SN761210

VIDEO-PROCESSING CIRCUIT

SOCS042 – JUNE 1994

4

POST OFFICE BOX 655303

detailed description

data in

Video data should be capacitively coupled into DATA IN.

sample and hold

A sample-and-hold pulse aligned to the video data should be applied to S/HD1.

correlated double sampling

The correlated-double-sampling pulse aligned to the output midlevel data should be applied to S/HP and S/HD2.

AGC section

For AGC with a maximum gain of 24 dB, AGCFIL should be set to 3 V and AGCMAX should be open (see

Figure 1). For AGC with a maximum gain of 11 dB, AGCMAX should be set to 0 V. A low-pass filter should be

placed between AGCOUT and AMPIN. A 0.1-

µF capacitor should be placed from AGCFB to GND. A line-clamp

pulse should be applied to HOBP.

0.5

2

AGCFIL Voltage – V

3.5

AGC GAIN CHANGE

vs

AGCFIL VOLTAGE

–6

26

– 2.48 dB

23.8 dB

Figure 1

gamma-amplifier section

AGC and low-pass filtered video should be brought into AMPIN. A 5-V voltage on GAINSEL results in the

filter-compensation amplifier having a 6-dB gain; a 0-V voltage on GAINSEL results in the filter compensation

amplifier having a 0-dB gain. A voltage that is applied to AMPTUNE determines the gamma curve (see Figure 2).

A high-control voltage on CHD allows the low-pass filtered and gamma-corrected video signal to pass through

to AMPOUT. A low-control voltage on CHD causes the 2-V level used for dark reference to be output. This

dark-level output either improves the last dark-reference clamp performance or provides a black-level reference

for inserting text characters. The accuracy of the charge stored across the last coupling capacitor is improved

since the random noise present on the CCD output during the optical-black level is eliminated.